page_title: Noise-immune cavity-enhanced optical heterodyne molecular spectroscopy
section_title: Modeling of signals
hierarchical_section_title: Noise-immune cavity-enhanced optical heterodyne molecular spectroscopy / Principles / Modeling of signals
caption_attribution_description: English: Author: Ove Axner, Unpublished figure
context_page_description: Noise-immune cavity-enhanced optical-heterodyne molecular spectroscopy is an ultra-sensitive laser-based absorption technique that utilizes laser light to assess the concentration or the amount of a species in gas phase by absorption spectrometry.
context_section_description: Frequency modulated Doppler-broadened signals can be modeled basically as ordinary fm-signals, although an extended description has to be used if the transition is optically saturated. Wavelength modulated Doppler broadened can be modeled by applying the conventional theory for wavelength modulation on the fm-signals.
  Since the electrical field in NICE-OHMS consists of three modes, a carrier and two sidebands, which propagate in positive and negative directions in the cavity, up to nine sub-Doppler signals can appear; four appearing at the absorption and five at the dispersion phase. Each of these signals can, in turn, originate from interactions between several groups of molecules with various pairs of modes (e.g. carrier-carrier, sideband-carrier, sideband-sideband in various combinations). In addition, since sub-Doppler signals necessarily involve optical saturation, each of these interactions has to be modeled by a more extensive description. This implies that the situation can be complex. In fact, there are still some types of sub-Doppler signals for which there so far are no adequate theoretical description.
caption: Typical (a) frequency modulated and (b) wavelength modulated Doppler-broadened NICE-OHMS signals from 13 ppb (10 μTorr, 13•10−9 atm) of C2H2. Individual markers: measured data; Solid curves: theoretical fits.
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